To reduce power consumption, circuit portions may be powered down when not in use, and powered up when they are again needed. Start-up circuitry is used to power-up desired circuit portions when needed. This mechanism increases battery life which is particularly desirable for portable electronic devices.
Previous start-up circuits include circuits having an NMOS transistor source tied to a node to be powered up and a drain tied to a power supply, such that the transistor is turned on if the source drops below a threshold which is established below the transistor gate voltage. The transistor is turned off if the source voltage becomes greater than the gate voltage plus the threshold voltage.
Other start-up circuits include those which provide current to a the node to be powered up and then detect when the circuit has powered up and thereafter cease supply of current to the node.
Circuits, for example mixed-signal circuits comprising analog-to-digital and digital-to-analog conversion functions, often require a reference voltage for operation. Voltage reference circuitry establishes the reference voltage when activated by an enabling signal. When high speed operation of a device is desirable it is advantageous to establish the reference voltage within a short duration of receiving the enabling signal. Start-up circuits have been used in conjunction with voltage reference circuits to improve response time.
FIG. 1 depicts a known start-up circuit 100 used in conjunction with a voltage reference circuit 102. Start-up circuit 100 is shown by dotted lines. Voltage reference circuit 102 has two possible equilibrium points, one of which corresponds to zero voltage and zero current, and a second, non-zero equilibrium point, which corresponds to a useful reference voltage. Therefore, voltage reference circuit 102 must be designed to choose only the non-zero equilibrium point to establish the reference voltage. Start-up circuit 100 is provided to allow voltage reference circuit 102 to utilize only the desired equilibrium point. If voltage reference circuit 102 is at the undesired equilibrium point, the voltage is zero and therefore, I.sub.1 and I.sub.2 are zero. Consequently, transistor 104 provides current in transistor 106 which then moves voltage reference circuit 102 to the non-zero equilibrium point. Transistor 104's source voltage increases as the desired equilibrium point is approached. This causes the current through transistor 104 to decrease. When voltage reference circuit 102 reaches the non-zero equilibrium point, the current through transistor 106 will be substantially the same as the current through transistor 108. Transistor 110 and resistor 112 set the gate bias voltage for transistor 104. Voltage reference circuit 102 is on within a gate bias voltage window. Therefore, the gate bias voltage must be high enough to turn voltage reference circuit 102 on but must not exceed the upper limit of the voltage window.
At startup, no current flows in bandgap circuit 204. Node 214 is pulled down by the kick-start circuit and node 212 is pulled up by the kick-start circuit. This causes current to flow in transistor 208 and by reflection in transistor 206 which holds node 212 above ground and this reflects current into the other branch to hold node 214 below the power supply and keep bandgap circuit 204 on. When current flows in the transistors of bankgap rteference circuit 204, kick-start circuit 202 is turned off. This occurs because transistor 205 mirrors the current in transistor 206 which drives the gate node of transistor 209 high and so pulls down node 211. Driving node 211 low turns off the current mirrors in kick-start circuit 202, so it stops sourcing and sinking current to the bandgap reference circuit 204. Resistor 210 ensures that current flows in kick-start circuit 202 when band gap reference circuit 204 is powered down.